Radiations, such as X-ray, gamma rays, infrared and visible light, as well as mechanical waves such as sonic and ultrasonic waves have been used to probe internal structures of an object. Radiography, mammographic imaging, ultrasound, Computed Tomography (CT), Positron Emitting Tomography (PET), and Magnetic Resonance Imaging (MRI) have been used widely for medical diagnostic purposes, in industrial measurements, security checks, and other areas.